Systems and methods for minimally-invasive optical-acoustic imaging

ABSTRACT

This document discusses, among other things, an imaging guidewire that includes one or more optical fibers communicating light along the guidewire. At or near its distal end, one or more blazed or other fiber Bragg gratings (FBGs) directs light to a photoacoustic transducer material that provides ultrasonic imaging energy. Returned ultrasound is sensed by an FBG sensor. A responsive signal is optically communicated to the proximal end of the guidewire, and processed to develop a 2D or 3D image. In one example, the guidewire outer diameter is small enough such that an intravascular catheter can be passed over the guidewire. Techniques for improving ultrasound reception include using a high compliance material, resonating the ultrasound sensing transducer, using an attenuation-reducing coating and/or thickness, and/or using optical wavelength discrimination. Techniques for improving the ultrasound generating transducer include using a blazed FBG, designing the photoacoustic material thickness to enhance optical absorption. Techniques for distinguishing plaque or vulnerable plaque may be used to enhance the displayed image.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/266,082, filed Oct. 7, 2002, now U.S. Pat. No. 7,245,789 whichapplication is incorporated herein by reference.

FIELD OF THE INVENTION

This document relates generally to imaging, and particularly, but not byway of limitation, to systems and methods for minimally-invasiveoptical-acoustic imaging.

BACKGROUND

Vardi et al. PCT Patent Application No. PCT/US99/04913, which publishedon 18 Nov. 1999 as International Publication No. WO 99/58059, discussesan optical-acoustic imaging device. In that document, which isincorporated herein by reference in its entirety, including itsdisclosure of a guidewire optical-acoustic imaging device, an elongatedimaging guidewire includes an elongated optical fiber driven by lightfrom a laser. A distal end of the guidewire includes a polyvinyldienefluoride (PVDF) film piezoelectric ultrasound transducer. The ultrasoundtransducer transmits ultrasound to an imaging region of interest aboutthe distal tip of the guidewire, and also receives the returnedultrasound energy. The received ultrasound energy deforms a Fiber BraggGrating (FBG or “Bragg grating”) at the distal end of the guide wire,which, in turn, modulates the optical signal through the optical fiber.Imaging information about the region of interest is then obtained at theproximal end of the guidewire from the modulated optical signal.

Among other things, the present applicant has recognized that apiezoelectric ultrasound transducer may be difficult to integrate with aminimally-invasive guidewire because of electrical signal losses inultrafine electrical conductors extending longitudinally through theguidewire assembly. Moreover, the present applicant has recognized thatthe field of view of the Vardi et al. device may be limited by the sizeof aperatures around the PVDF ultrasound transducers and/or the spacingbetween FBGs. Furthermore, the present applicant has recognized that thesensitivity of the Vardi et al. device may limit its usefulness in animaging application. For these and other reasons, the present applicanthas recognized that there is an unmet need in the art for improvedsystems and methods for performing optical-acoustic imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsdescribe substantially similar components throughout the several views.Like numerals having different letter suffixes represent differentinstances of substantially similar components. The drawings illustrategenerally, by way of example, but not by way of limitation, variousembodiments discussed in the present document.

FIG. 1 is a cross-sectional side view illustrating generally, by way ofexample, but not by way of limitation, one example of an FBG strainsensor in an optical fiber.

FIG. 2 is a cross-sectional side view illustrating generally, by way ofexample, but not by way of limitation, an example of an FBG gratinginterferometer sensor.

FIG. 3A is a cross-sectional schematic diagram illustrating generallyone example of a distal portion of an imaging guidewire that combines anacousto-optic FBG sensor with an photoacoustic transducer.

FIG. 3B is a cross-sectional schematic diagram illustrating generallyone example of a distal portion of an imaging guidewire that combines anacousto-optic FBG sensor with an photoacoustic transducer.

FIG. 4 is a cross-sectional schematic diagram illustrating generally oneexample of the operation of a blazed grating FBG photoacoustictransducer.

FIG. 5 is a schematic diagram comparing an expected angular sensitivitypattern of an exemplary blazed FBG optical-to-acoustic andacoustic-to-optical combined device to that of a piezoelectric sensor.

FIG. 6 is a schematic diagram illustrating generally one technique ofgenerating an image by rotating the blazed FBG optical-to-acoustic andacoustic-to-optical combined transducer and displaying the resultantseries of radial image lines to create a radial image.

FIG. 7 is a schematic diagram that illustrates generally one such phasedarray example, in which the signal to/from each array transducer iscombined with the signals from the other transducers to synthesize aradial image line.

FIG. 8 is a schematic diagram that illustrates generally an example of aside view of a distal portion of a guidewire.

FIG. 9 is a schematic diagram that illustrates generally one example ofa cross-sectional side view of a distal portion of a guidewire.

FIG. 10 is a schematic diagram that illustrates generally one example ofa cross-sectional end view of a proximal portion of a guidewire.

FIG. 11 is a schematic diagram that illustrates generally one example ofa cross-sectional end view of a distal portion of a guidewire.

FIG. 12 is a schematic diagram that illustrates generally one example ofa cross-sectional side view of a distal portion of a guidewire.

FIG. 13A is a cross-sectional schematic diagram illustrating generallyone example of a proximal portion of a guidewire, which iscommunicatively coupled to an instrumentation/control interface via anoptical coupler.

FIG. 13B is a cross-sectional schematic diagram illustrating generally afurther example of a proximal portion of a guidewire that iscommunicatively coupled to an instrumentation/control interface using anoptical coupler.

FIG. 14A is a block diagram illustrating generally one example of theimaging guidewire and associated interface components.

FIG. 14B is a block diagram illustrating generally another example ofthe imaging guidewire and associated interface components, includingtissue characterization and image enhancement modules.

FIG. 15 is a cross-sectional schematic diagram illustrating generallyone example of an alternate acoustic-to-optical transducer.

FIG. 16 is a cross-sectional schematic diagram illustrating generallyone example of operation of the acoustic-to-optical transducer of FIG.15.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the scope of the presentinvention. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined by the appended claims and their equivalents.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one. Furthermore, allpublications, patents, and patent documents referred to in this documentare incorporated by reference herein in their entirety, as thoughindividually incorporated by reference. In the event of inconsistentusages between this documents and those documents so incorporated byreference, the usage in the incorporated reference(s) should beconsidered supplementary to that of this document; for irreconcilableinconsistencies, the usage in this document controls. In this document,the term “minimally-invasive” refers to techniques that are lessinvasive than conventional surgery; the term “minimally-invasive” is notintended to be restricted to the least-invasive technique possible.

1. Examples of Fiber Bragg Grating Acousto-Optic Sensors

FIG. 1 is a cross-sectional side view illustrating generally, by way ofexample, but not by way of limitation, one example of a strain-detectingFBG sensor 100 in an optical fiber 105. FBG sensor 100 senses acousticenergy received from a nearby area to be imaged, and transduces thereceived acoustic energy into an optical signal within optical fiber105. In the example of FIG. 1, FBG sensor 100 includes Bragg gratings110A-B in an optical fiber core 115 surrounded by an optical fibercladding 120. Bragg gratings 110A-B are separated by a strain sensingregion 125, which, in one example, is about a millimeter in length. Thisexample senses strain by detecting an “optical displacement” betweenthese gratings 110A-B.

A fiber Bragg grating can be conceptualized as a periodic change in theoptical index (which is inversely proportional to the speed of light inthe material) of a portion of the optical fiber core 115. Light of aspecific wavelength traveling down such a portion of core 115 will bereflected; the period (distance) 130 of the change in the optical indexdetermines the particular wavelength of light that will be reflected.The degree of index change and the length 135 of the grating determinethe ratio of light reflected to that transmitted through the grating.

FIG. 2 is a cross-sectional side view illustrating generally, by way ofexample, but not by way of limitation, an operative example of aninterferometric FBG sensor 100. The example of FIG. 2 includes two FBGs110A-B. FBG 110A is partially reflective at a specific wavelength oflight passing through fiber core 115. FBG 110B is substantially fullyreflective of such light. This interferometric arrangement of FBGs110A-B is capable of discerning the “optical distance” between FBGs110A-B with extreme sensitivity. The “optical distance” is a function ofthe velocity of light in the material of fiber core 115 as well as thelength 125 between FBGs 110A-B. Thus, a change in the velocity of lightcan induce a change in optical distance even though the physicaldistance 125 between FBGs 110A-B has not changed.

An interferometer such as FBG sensor 100 can be conceptualized as adevice that measures the interference between two paths taken by anoptical beam. A partially reflecting FBG 110A (or a partially reflectingmirror) is used to split the incident beam of light into two parts. Inan interferometer, one part of the beam travels along a path that iskept constant (i.e., a control path) and the other part travels a pathwhere some change is to be monitored (i.e., a test path). Usingpartially reflecting FBG 110A (or a partially reflecting mirror, eitherof which may alternatively be in addition to FBG 110A), the two parts ofthe beam are combined. If the two paths are identical, the parts combineto form the original beam. If the two paths are different, the two partswill add or subtract from each other. This addition or subtraction isknown as interference. A complete subtraction is called a null andoccurs at a precise wavelength of light for a given difference in paths.Measuring the wavelength where this null occurs yields an indication ofthe difference in optical paths between the two beams. In such a manner,an interferometer such as FBG sensor 100 senses small changes indistance, such as a change in the optical distance 125 between FBGs110A-B resulting from received ultrasound or other received acousticenergy.

In one example, such as illustrated in FIG. 2, the interferometric FBGsensor 100 causes the interference between that portion of the opticalbeam that is reflected off the first (partially reflective) FBG 110Awith that reflected from the second (substantially fully reflective) FBG110B. The wavelength of light where an interferometric null will occuris very sensitive to the “optical distance” 125 between the two FBGs110A-B. This interferometric FBG sensor 100 of FIG. 2 has another verypractical advantage. In this example, the two optical paths along thefiber core 115 are the same, except for the sensing region between FBGs110A-B. This shared path ensures that any optical changes in the sharedportion of optical fiber 105 will have substantially no effect upon theinterferometric signal; only the change in the sensing region betweenFBGs 125 is sensed.

2. Examples of Devices and Methods Improving FBG Reception

In one example, an FBG sensor 100 senses strain generated by ultrasoundor other acoustic energy received from a nearby imaging region to bevisualized and, in response, modulates an optical signal in an opticalfiber. Increasing the sensitivity of the FBG sensor 100 providesimproved imaging. A first example of increasing sensitivity is toincrease the amount of strain induced in the FBG sensor 100 for a givendynamic pressure provided by the acoustic energy. A second example is toincrease the modulation of the optical signal for a given change instrain of the FBG sensor 100.

One technique of increasing the strain induced in the FBG sensor 100 isto design the physical attributes of the FBG sensor 100 to increase thedegree of strain for a given externally applied acoustic field. In onesuch example, increased strain is obtained by using material that has ahigh degree of strain for a given stress. Calculations indicate thatover two orders of magnitude increase in strain vs. stress (alsoreferred to as “compliance”) is obtained by using an optical gradeplastic, rather than glass, in the fiber core 115 of the FBG sensor 100.One example of a suitable optical grade plastic used in fiber core 115is poly-methyl-methacrylate (PMMA).

In a second example, the FBG sensor 100 is shaped so as to increase thestrain for a given applied acoustic pressure field. FIG. 3A is across-sectional schematic diagram illustrating one such example in whichthe FBG sensor 100 is shaped such that it mechanically resonates at thefrequency of the acoustic energy received from the nearby imagingregion, thereby resulting in increased strain. In the example of FIG.3A, all or a portion of the strain sensing region between FBGs 110A-B isselected to provide a resonant thickness 300 that promotes mechanicalresonance of the received acoustic energy, thereby increasing theresulting strain sensed by FBG sensor 100. In one example, such asillustrated in FIG. 3A, this is accomplished by grinding or otherwiseremoving a portion of fiber cladding 120, such that the remainingthickness of fiber core 115 and/or fiber cladding 120 between opposingplanar (or other) surfaces is selected to mechanically resonate at thefrequency of the acoustic energy received from the nearby imagingregion.

In one example, for a particular material, mechanical resonance isobtained by making the resonant thickness 300 of the strain sensingregion substantially the same thickness as ½ the acoustic wavelength (oran odd integer multiple thereof) in the material(s) of FBG sensor 100 atthe acoustic center frequency of the desired acoustic frequency bandreceived from the imaging region. In other examples, such as for othermaterials, the resonant thickness 300 is selected to match a differentproportion of the acoustic wavelength that obtains the desiredmechanical resonance for that material. Calculations indicate thatobtaining such mechanical resonance will increase the strain sensitivityby about an order of magnitude over that of a sensor that is notconstructed to obtain such mechanical resonance.

In a third example, a special coating 305 is applied to the FBG sensor100 to increase the acoustic pressure as seen by the FBG sensor 100 overa band of acoustic frequencies, thereby improving its sensitivity overthat band. The difference between the mechanical characteristics ofwater (or tissue and/or blood, which is mostly comprised of water) andglass material of the optical fiber 105 carrying the FBG sensor 100 istypically so significant that only a small amount of acoustic energy“enters” the FBG sensor 100 and thereby causes strain; the remainingenergy is reflected back into the biological or other material beingimaged. For a particular range of acoustic frequencies, one or morecoatings 305 of specific thickness 310 and/or mechanical properties(e.g., the particular mechanical impedance) of the coating material candramatically reduce such attenuation due to the different mechanicalcharacteristics. One example uses quarter wave matching, providing acoating 305 of a thickness 310 that is approximately equal to onequarter of the acoustic signal wavelength received from the region beingimaged. Using such matching, the sensitivity of the FBG sensor 100, overa given band of acoustic frequencies of interest, is expected toincrease by about an order of magnitude.

In one example, using the above-discussed quarter wave matching andsensor shaping techniques, the sensitivity of the FBG sensor 100approaches that of a piezoelectric transducer. Additionally usingoptical grade plastic for fiber core 115, in conjunction with one or theother of these techniques, will further increase the sensitivity of theFBG sensor 100.

Additionally (or alternatively) to the above techniques of increasingthe strain of the FBG sensor 100 for a particular level of acousticenergy, in one example, the optical sensitivity of the FBG sensor 100 tostrain is increased, thereby increasing the sensitivity of the FBGsensor 100 to an acoustic field. In one example, this is accomplished byimproved techniques of optical wavelength discrimination, such as byusing a fiber-based Mach-Zehnder interferometer or by construction ofimproved optical fiber geometries that increase the optical sensitivityof the FBG sensor 100 to strain.

3. Examples of Systems and Methods Improving Ultrasound Transmission

The present applicant has recognized that while it may be possible toimplement an imaging guidewire that transmits ultrasound using apiezoelectric transducer, such a design may involve a trade-off. If thepiezoelectric transducer radiates the ultrasonic energy in a broadradial pattern, imaging quality may be degraded. Conversely, usingsmaller piezoelectric transducers to transmit ultrasound may requiresignificant electrical voltages in a guidewire in order to achieveneeded acoustic transmit energy. This is because a smaller transducerhas a higher electrical impedance, needing a higher voltage to achievethe same acoustic power. Such a guidewire must also use materials ofsufficient dielectric properties to ensure patient safety. Moreover,adding electrically conducting wires to a guidewire assembly complicatesits manufacture.

However, optical energy can be converted to acoustic energy. In oneexample, therefore, such problems are overcome using anoptical-to-acoustic transmitter, which, in one example, is integratedwith an acoustic-to-optical receiver such as FBG sensor 100.

FIG. 3B is a cross-sectional schematic diagram illustrating generallyone example of a distal portion of an imaging guidewire that combines anacousto-optic FBG sensor 100 with an photoacoustic transducer 325. Inthis example, photoacoustic transducer 325 includes a blazed Bragggrating 330. In the illustrative example of FIG. 3B, blazed Bragggrating 330 is implemented in the strain sensitive region of the FBGsensor 100, between FBGs 110A-B, however, this is not a requirement.Unlike an unblazed Bragg grating, which typically includes impressedindex changes that are substantially perpendicular to the longitudinalaxis of the fiber core 115 of the optical fiber 105, the blazed Bragggrating 330 includes obliquely impressed index changes that are at anonperpendicular angle to the longitudinal axis of the optical fiber105.

A standard unblazed FBG partially or substantially fully reflectsoptical energy of a specific wavelength traveling down the axis of thefiber core 115 of optical fiber 105 back up the same axis. Blazed FBG330 reflects this optical energy away from the longitudinal axis of theoptical fiber 105. For a particular combination of blaze angle andoptical wavelength, the optical energy will leave blazed FBG 330substantially normal (i.e., perpendicular) to the longitudinal axis ofthe optical fiber 105. In the illustrative example of FIG. 3B, anoptically absorptive photoacoustic material 335 (also referred to as a“photoacoustic” material) is placed on the surface of optical fiber 105.The optically absorptive photoacoustic material 335 is positioned, withrespect to the blazed grating 330, so as to receive the optical energyleaving the blazed grating. The received optical energy is converted inthe optically absorptive material 335 to heat that expands the opticallyabsorptive photoacoustic material 335. The optically absorptivephotoacoustic material 335 is selected to expand and contract quicklyenough to create and transmit an ultrasound or other acoustic wave thatis used for acoustic imaging of the region of interest about the distaltip (or other desired portion) of the imaging guidewire. In one example,the optically absorptive photoacoustic material 335 is the same materialas the acoustic matching material 305 discussed above.

FIG. 4 is a cross-sectional schematic diagram illustrating generally oneexample of the operation of photoacoustic transducer 325 using a blazedBragg grating 330. Optical energy of a specific wavelength, λ₁, travelsdown the fiber core 115 of optical fiber 105 and is reflected out of theoptical fiber 105 by blazed grating 330. The outwardly reflected opticalenergy impinges on the photoacoustic material 335. The photoacousticmaterial 335 then generates a responsive acoustic impulse that radiatesaway from the photoacoustic material 335 toward nearby biological orother material to be imaged. Acoustic energy of a specific frequency isgenerated by optically irradiating the photoacoustic material 335 at apulse rate equal to the desired acoustic frequency.

In another example, the photoacoustic material 335 has a thickness 340(in the direction in which optical energy is received from blazed Bragggrating 330) that is selected to increase the efficiency of emission ofacoustic energy. In one example, thickness 340 is selected to be about ¼the acoustic wavelength of the material at the desired acoustictransmission/reception frequency. This improves the generation ofacoustic energy by the photoacoustic material.

In yet a further example, the photoacoustic material is of a thickness300 that is about ¼ the acoustic wavelength of the material at thedesired acoustic transmission/reception frequency, and the correspondingglass-based optical fiber sensing region resonant thickness 300 is about½ the acoustic wavelength of that material at the desired acoustictransmission/reception frequency. This further improves the generationof acoustic energy by the photoacoustic material and reception of theacoustic energy by the optical fiber sensing region.

In one example of operation, light reflected from the blazed gratingexcites the photoacoustic material in such a way that the optical energyis efficiently converted to substantially the same acoustic frequencyfor which the FBG sensor is designed. The blazed FBG and photoacousticmaterial, in conjunction with the aforementioned FBG sensor, provideboth a transmit transducer and a receive sensor, which are harmonized tocreate an efficient unified optical-to-acoustic-to-opticaltransmit/receive device. In one example, the optical wavelength forsensing is different from that used for transmission. In a furtherexample, the optical transmit/receive frequencies are sufficientlydifferent that the reception is not adversely affected by thetransmission, and vice-versa.

FIG. 5 is a schematic diagram comparing an expected angular sensitivitypattern of an exemplary blazed FBG optical-to-acoustic andacoustic-to-optical combined device 500 to that of a piezoelectrictransducer 505. As seen in the example of FIG. 5, theoptical-to-acoustic-to-optical sensor assembly 500 is expected to becapable of operating over a specific angular range that is substantiallysimilar to that of the piezoelectric transducer 505 of similardimensions. Therefore, in one example, the blazed FBGoptical-to-acoustic and acoustic-to-optical combined device 500 iscapable of using conventional intravascular ultrasound (“IVUS”)techniques.

FIG. 6 is a schematic diagram illustrating generally one technique ofgenerating an image of a vessel wall 600 by rotating the blazed FBGoptical-to-acoustic and acoustic-to-optical combined transducer 500 anddisplaying the resultant series of radial image lines to create a radialimage. In another example, phased array mages are created using asubstantially stationary (i.e., non-rotating) set of multiple FBGsensors, such as FBG sensors 500A-J. FIG. 7 is a schematic diagram thatillustrates generally one such phased array example, in which the signalto/from each array transducer 500A-J is combined with the signals fromone or more other transducers 500A-J to synthesize a radial image line.In this example, other image lines are similarly synthesized from thearray signals, such as by using specific changes in the signalprocessing used to combine these signals.

4. Examples of Guidewire Design

FIG. 8 is a schematic diagram that illustrates generally an example of aside view of a distal portion 800 of an imaging guidewire 805 or otherelongate catheter (in one example, the guidewire 805 is capable of beingused for introducing and/or guiding a catheter or other medicalinstrument, e.g., over the guidewire 805). In this example, the distalportion 800 of the imaging guidewire 805 includes one or more imagingwindows 810A, 810B, . . . , 810N located slightly or considerablyproximal to a distal tip 815 of the guidewire 805. Each imaging window810 includes one or more optical-to-acoustic transducers 325 and acorresponding one or more separate or integrated acoustic-to-optical FBGsensors 100. In one example, each imaging window 810 includes an arrayof blazed FBG optical-to-acoustic and acoustic-to-optical combinedtransducers 500 (such as illustrated in FIG. 7) located slightlyproximal to distal tip 815 of guidewire 805 having mechanical propertiesthat allow the guidewire 805 to be guided through a vascular or otherlumen. In one example, the different imaging windows 810A, 810B, . . . ,810N are designed for different optical wavelengths, such thatindividual windows can be easily addressed by changing the opticalwavelength being communicated through fiber core 115.

FIG. 9 is a schematic diagram that illustrates generally one example ofa cross-sectional side view of a distal portion 900 of another guidewire905. In this example, the guide-wire 905 includes a solid metal or othercore 910 that tapers down in diameter (e.g., from an outer diameter ofabout 0.011 inches) at a suitable distance 915 (e.g., about 50 cm) fromthe distal tip 920, to which the tapered core 910 is attached. In thisexample, optical fibers 925 are distributed around the outercircumference of the guidewire core 910, and attached to the distal tip920. In this example, the optical fibers 925 are at least partiallyembedded in a polymer matrix or other binder material that bonds theoptical fibers 925 to the guidewire core 910 and/or the distal tip 920.The binder material may also contribute to the torsion response of theresulting guidewire assembly 905. In one example, the optical fibers 925and binder material is overcoated with a polymer or other coating 930,such as for providing abrasion resistance, optical fiber protection,and/or friction control. In this example, the composite structure of thedistal region 900 of the guidewire 905 provides, among other things,flexibility and rotational stiffness, thereby allowing the guidewire 905to be maneuvered to an imaging region of interest within a vascular orany other lumen.

FIG. 10 is a schematic diagram that illustrates generally one example ofa cross-sectional end view of a proximal portion 1000 of guidewire 905,which includes guidewire core 910, optical fibers 925, binder material1005, and outer coating 930. In this example, but not by way oflimitation, the diameter of the core 910 is about 11/1000 inch, thediameter of the optical fibers 925 is about (1.25)/1000 inch, and theoptional outer coating 930 is about (0.25)/1000 inch thick.

FIG. 11 is a schematic diagram that illustrates generally one example ofa cross-sectional end view of distal portion 900 of guidewire 905, e.g.,adjacent to distal tip 920. In this example, but not by way oflimitation, the diameter of core 910 has tapered down to about(1.5)/1000 inch, circumferentially surrounded by a void 1100 of aboutthe same diameter (e.g., about 11/1000 inch) as the core 910 near theproximal end 100 of the guidewire 905. In this example, the opticalfibers 925 are circumferentially disposed in the binder material 1005around the void 1100. Binder material 1005 provides structural support.Optical fibers 925 are optionally overlaid with the outer coating 930.

FIG. 12 is a schematic diagram that illustrates generally one example ofa cross-sectional side view of a distal portion 900 of a guidewire 905.In this example, at least one metallic or other bulkhead 1200 isprovided along the tapered portion of the guidewire core 910. Theoptical fibers 925 and binder 1005 are attached to a proximal side ofthe bulkhead 1200 near its circumferential perimeter. A distal side ofthe bulkhead 1200 is attached, near its circumferential perimeter, to acoil winding 1205 that extends further, in the distal direction, to aball or other distal tip 920 of the guidewire 905.

5. Examples of Acoustic Transducer Construction

In one example, before the acoustic transducer(s) is fabricated, theguidewire 905 is assembled, such as by binding the optical fibers 925 tothe core 910 and distal tip 920 or bulkhead 1200, and optionally coatingthe guidewire 905. The optoacoustic transducer(s) are then integratedinto the guidewire assembly 905, such as by grinding one or more groovesin the guidewire at the locations of the optoacoustic transducer windows810. In a further example, the depth of these groove(s) in the opticalfiber(s) 925 defines the resonant structure(s) of the optoacoustictransducer(s).

After the optoacoustic transducer windows 810 have been defined, theFBGs added to one or more portions of the optical fiber 925 within suchwindows 810. In one example, the FBGs are created using an opticalprocess in which the portion of the optical fiber 925 is exposed to acarefully controlled pattern of UV radiation that defines the Bragggratings. Then, a photoacoustic material is deposited or otherwise addedin the transducer windows 810 over respective Bragg gratings. Oneexample of a suitable photoacoustic material is pigmentedpolydimethylsiloxane (PDMS), such as a mixture of PDMS, carbon black,and toluene. Thus, in this example, the FBGs are advantageouslyconstructed after the major elements of the guidewire are mechanicallyassembled into the guidewire assembly 905.

6. Examples of Proximal End Interface

In one example, the guidewire 905 allows for over-the-guidewire or otherinsertion of a catheter at the proximal end of the guidewire. Therefore,in such an example, the guidewire 905 (including any proximal endinterface) has an outer diameter that is less than or equal to the innerdiameter (e.g., 0.014 inches) of a catheter to allow the catheter with asimilarly-sized inner diameter to travel over the guidewire 905. FIGS.13A and 13B provide illustrative examples of an optical coupler thateasily engages and disengages guidewire 905. Among other things, thisfacilitates over-the-guidewire catheter insertion, and viewing animaging region either before, during, or after such a catheter isinserted over-the-guidewire.

FIG. 13A is a cross-sectional schematic diagram illustrating generallyone example of a proximal portion 1300 of guidewire 905, which iscommunicatively coupled to an instrumentation/control interface via anoptical coupler 1305. In this example, proximal portion 1300 ofguidewire 905 is received within a receptacle 1310 portion of opticalcoupler 1305, and includes one or more blazed FBGs 1315 to couple lightinto and/or out of one or more respective optical fibers 925 ofguidewire 905. Optical coupler 1305 includes one or more correspondingblazedFBGs 1320 to couple light into and/or out of one or morerespective optical fibers 1325 of optical coupler 1305. In the exampleof FIG. 13A, the FBGs 1320 of optical coupler 1305 are locatedsubstantially adjacent to corresponding FBGs 1315 of guidewire 905 whenguidewire 905 is engaged within receptacle 1310 of optical coupler 1305.Although FIG. 13A illustrates a multiple-fiber embodiment of guidewire905, the illustrated techniques for coupling to aninstrumentation/control interface are also applicable to a guidewirethat includes a single optical fiber.

FIG. 13B is a cross-sectional schematic diagram illustrating generally afurther example of a proximal portion 1300 of guidewire 905 that iscommunicatively coupled to an instrumentation/control interface using anoptical coupler 1305. In the example of FIG. 13B, at least one opticalfiber 925 transmits light at a different wavelength from that at whichit receives light. Therefore, such an optical fiber 925 includes twoseparate blazed FBGs that couple light into and out of each such opticalfiber 925. For example, as illustrated in FIG. 13B, optical fiber 925Aincludes a first blazed FBG 1315A operating at the transmit wavelength,and a second blazed FBG 1330A operating at the receive wavelength.Optical coupler 1305 includes a corresponding first blazed FBG 1320Aoperating at the transmit wavelength and a second blazed FBG 1335Aoperating at the receive wavelength. When a proximal portion 1300 ofguidewire 905 is fully inserted into receptacle 1310, blazed FBGs 1320Aand 1315A are located substantially adjacent to each other, and blazedFBGs 1335A and 1330A are located substantially adjacent to each other.Similarly, optical fiber 925B and optical coupler 1305 respectivelyinclude substantially adjacent transmit FBGs 1315B and 1320B andsubstantially adjacent receive FBGs 1330B and 1335B.

For additional optoacoustic transducer windows 810 at or near the distalportion 900 or elsewhere along guidewire 905, corresponding additionalblazed FBGs may similarly be included on optical coupler 1305 at theappropriate wavelength for transmitting and/or receiving optical energywith respect to such additional optoacoustic transducer windows 810.Moreover, optical coupler 1305 need not be located exactly at theproximal end of the guidewire 905, but may instead be located anywherenear the proximal portion 1300 of the guidewire 905 or even furthertoward the distal portion 900 of the guidewire 905. Also, alignment ofthe optical coupler 1305 to guidewire 905 need not be limited to buttingguidewire 905 into receptacle 1310 of optical coupler 1305; any otheralignment mechanism and/or technique is also included.

7. Examples of Process and Control Imaging Electronics

FIG. 14A is a block diagram illustrating generally one example of theimaging guidewire 905 and associated interface components. The blockdiagram of FIG. 14A includes the imaging guidewire 905, which is coupledby optical coupler 1305 to an optoelectronics module 1400. Theoptoelectronics module 1400 is coupled to an image processing module1405 and a user interface 1410 that includes a display providing aviewable still and/or video image of the imaging region near one or moreacoustic-to-optical transducers using the acoustically-modulated opticalsignal received therefrom. In one example, the system 1415 illustratedin the block diagram of FIG. 14A uses an image processing module 1405and a user interface 1410 that are substantially similar to existingacoustic imaging systems.

FIG. 14B is a block diagram illustrating generally another example ofthe imaging guidewire 905 and associated interface components. In thisexample, the associated interface components include a tissue (andplaque) characterization module 1420 and an image enhancement module1425. In this example, an input of tissue characterization module 1420is coupled to an output from optoelectronics module 1400. An output oftissue characterization module 1420 is coupled to at least one of userinterface 1410 or an input of image enhancement module 1425. An outputof image enhancement module 1425 is coupled to user interface 1410, suchas through image processing module 1405.

In this example, tissue characterization module 1420 processes a signaloutput from optoelectronics module 1400. In one example, such signalprocessing assists in distinguishing plaque from nearby vascular tissue.Such plaque can be conceptualized as including, among other things,cholesterol, thrombus, and loose connective tissue that build up withina blood vessel wall. Calcified plaque typically reflects ultrasoundbetter than the nearby vascular tissue, which results in high amplitudeechoes. Soft plaques, on the other hand, produce weaker and moretexturally homogeneous echoes. These and other differencesdistinguishing between plaque deposits and nearby vascular tissue aredetected using tissue characterization signal processing techniques.

For example, such tissue characterization signal processing may includeperforming a spectral analysis that examines the energy of the returnedultrasound signal at various frequencies. A plaque deposit willtypically have a different spectral signature than nearby vasculartissue without such plaque, allowing discrimination therebetween. Suchsignal processing may additionally or alternatively include statisticalprocessing (e.g., averaging, filtering, or the like) of the returnedultrasound signal in the time domain. Other signal processing techniquesknown in the art of tissue characterization may also be applied. In oneexample, the spatial distribution of the processed returned ultrasoundsignal is provided to image enhancement module 1425, which providesresulting image enhancement information to image processing module 1405.In this manner, image enhancement module 1425 provides information touser interface 1410 that results in a displaying plaque deposits in avisually different manner (e.g., by assigning plaque deposits adiscernable color on the image) than other portions of the image. Otherimage enhancement techniques known in the art of imaging may also beapplied. In a further example, similar techniques are used fordiscriminating between vulnerable plaque and other plaque, and enhancingthe displayed image provide a visual indicator assisting the user indiscriminating between vulnerable and other plaque.

8. Examples of Opto-Electronics Module

The opto-electronics module 1400 may include one or more lasers andfiber optic elements. In one example, such as where different transmitand receive wavelengths are used, a first laser is used for providinglight to the guidewire 905 for the transmitted ultrasound, and aseparate second laser is used for providing light to the guidewire 905for being modulated by the received ultrasound. In this example, a fiberoptic multiplexer couples each channel (associated with a particular oneof the guidewire's optical fibers 925) to the transmit and receivelasers and associated optics. This reduces system complexity and costs.

In one example, the sharing of transmit and receive components bymultiple guidewire channels is possible at least in part because theacoustic image is acquired over a relatively short distance (e.g.,millimeters). The speed of ultrasound in a human or animal body is slowenough to allow for a large number of transmit/receive cycles to beperformed during the time period of one image frame. For example, at animage depth (range) of about 2 cm, it will take ultrasonic energyapproximately 26 microseconds to travel from the sensor to the rangelimit, and back. In one such example, therefore, an about 30microseconds transmit/receive (T/R) cycle is used. In the approximately30 milliseconds allotted to a single image frame, up to 1,000 T/R cyclescan be carried out. In one example, such a large number of T/R cyclesper frame allows the system to operate as a phased array even thougheach sensor is accessed in sequence. Such sequential access of thephotoacoustic sensors in the guidewire permits (but does not require)the use of one set of T/R opto-electronics in conjunction with asequentially operated optical multiplexer.

9. Example of Use for 3-Dimensional (3-D) Imaging

In one example, instead of presenting one 2-D slice of the anatomy, thesystem is operated to provide a 3-D visual image that permits theviewing of a desired volume of the patient's anatomy or other imagingregion of interest. This allows the physician to quickly see thedetailed spatial arrangement of structures, such as lesions, withrespect to other anatomy. In one example, in which the guidewire 905includes 30 sequentially-accessed optical fibers having up to 10photoacoustic transducer windows per optical fiber, 30×10=300 T/R cyclesare used to collect the image information from all the transducerwindows for one image frame. This is well within the allotted 1,000 suchcycles for a range of 2 cm, as discussed above. Thus, such an embodimentallows substantially simultaneous images to be obtained from all 10transducer windows at of each optical fiber at video rates (e.g., atabout 30 frames per second for each transducer window). This allowsreal-time volumetric data acquisition, which offers a distinct advantageover other imaging techniques. Among other things, such real-timevolumetric data acquisition allows real-time 3-D vascular imaging,including visualization of the topology of a blood vessel wall, theextent and precise location of plaque deposits, and, therefore, theability to identify vulnerable plaque.

10. Alternate Example of Acoustic-to-Optical Receiver

FIG. 15 is a cross-sectional schematic diagram illustrating generallyone example of an alternate acoustic-to-optical transducer 1500, whichin this example is integrated into an optical fiber 105, including fibercore 105 and fiber cladding 120 and covered by coating 930. In theillustrative example of FIG. 15, transducer 1500 includes a blazed FBG330 in core 115, a translucent deformable (or empty) region 1505 incladding 120, and an acoustically-deformable light-reflective surfaceregion 1510 overlaying at least a portion of translucent region 1505. Inone example, acoustic-to-optical transducer 1500 is fabricated in awindow 810 of an imaging guidewire 805 along with an optical-to-acoustictransducer 325, which generates acoustic energy in a nearby imagingregion of interest to be received by acoustic-to-optical transducer1500.

FIG. 16 is a cross-sectional schematic diagram illustrating generallyone example of acoustic-to-optical transducer 1500 in operation. FBG 330receives light from a proximal end of fiber core 105, and directs thereceived light outward through translucent region 1505 such that thelight impinges upon, and is reflected by, reflective region 1510. Atleast some of the reflected light is received at FBG 330 and directedback toward the proximal end of fiber core 105. As illustrated in FIG.16, reflective region 1510 deflects in response to acoustic energyreceived from the nearby imaging region of interest as a result ofinsonification by a nearby optical-to-acoustic transducer 325. Thedeflection of reflective region 1510 modulates the distance that thelight travels between FBG 330 and reflective region 1510. The resultingchange in wavelength or intensity is monitored by interfaceoptoelectronics coupled to a proximal end of optical fiber 105, such asusing the above-described components and techniques.

As illustrated in the example of FIGS. 15 and 16, acoustic-to-opticaltransducer 1500 need only include a single FBG (e.g., blazed FBG 330).Moreover, acoustic-to-optical transducer 1500 need not rely on thePoisson effect in which received acoustic energy “squeezes” in a firstdirection, thereby modulating an interferometric strain-sensing distancein a second direction that is normal to the first direction. Anacoustic-to-optical transducer using the Poisson effect typicallysuffers from some attenuation in translating the mechanical force fromthe first direction to the orthogonal second direction. As illustratedin FIG. 16, however, the acoustic-to-optical transducer 1500 detects amodulating distance that is in the substantially the same direction asthe received acoustic energy. Moreover, because region 1510 isreflective, a given deflection results in a modulation of twice thenumber of wavelengths of light in that deflection distance. This furtherincreases the sensitivity of acoustic-to-optical transducer 1500.

In one example, region 1505 is filled with a transparent polymer toallow optical energy to pass through. In a further example, region 1505has a thickness 1515 that is ¼-wave resonant with the received acousticpressure wave. In such an example, the resonance of the polymer-filledregion 1505 serves to increase the motion of the reflective region 1510over that which would occur if region 1505 were formed of glass. In afurther example, the polymer-filled region 1505 includes an acousticimpedance that is close to that of water and, therefore, human or animaltissue.

11. CONCLUSION

Although certain of the above examples have been described with respectto intravascular imaging (e.g., for viewing and/or identifyingvulnerable plaque), the present systems, devices, and methods are alsoapplicable to imaging any other body part. For example, for exampleguidewire or other elongate body as discussed above could be insertedinto a biopsy needle, laparoscopic device, or any other lumen or cavityfor performing imaging. Moreover, such imaging need not involveinsertion of an elongate body into a lumen, for example, an imagingapparatus could alternatively be wrapped around a portion of a region tobe imaged.

In another example, this technology can be used to process the Dopplershift in acoustic frequency to image blood flow. The operation would besimilar to that described above, however, this would increase the lengthof the transmitted acoustic signal, and would use known Doppler signalprocessing in the image processing portion of the control electronics.The transmitted acoustic signal can be lengthened by repeatedly pulsingthe transmit optical energy at the same rate as the desired acousticfrequency.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments may be used in combination with each other. Many otherembodiments will be apparent to those of skill in the art upon reviewingthe above description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled. In the appendedclaims, the terms “including” and “in which” are used as theplain-English equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements on their objects.

1. An imaging apparatus including: an elongated body having proximal anddistal ends, the body including at least one optical fiber; anoptical-to-acoustic transducer, near the distal end of the body,configured to generate acoustic energy for imaging a region near thedistal end of the body in response to a first optical signal in theoptical fiber, wherein the optical-to-acoustic transducer includes aphotoacoustic material that is located substantially outside of a coreof the optical fiber but that is capable of receiving light from thecore of the optical fiber to generate an optical-to-acoustic response;an acoustic-to-optical transducer near the distal end of the body,configured to sense acoustic energy from the region near the distal endof the body and to provide a responsive second optical signal in theoptical fiber; and a user interface, including a display configured toprovide an image of the region near the distal end of the body using thesecond optical signal.
 2. The apparatus of claim 1, comprising means fordirecting light passing along the core out of the core and into thephotoacoustic material.
 3. The apparatus of claim 1, comprising a blazedBragg grating located in a core of the optical fiber, in which theblazed Bragg grating redirects light passing axially along the core outof the core and into the photoacoustic material.
 4. The apparatus ofclaim 3, in which the elongated body provides a guidewire sized andshaped for at least one of introducing a catheter into a lumen andguiding a catheter within a lumen.
 5. The apparatus of claim 4, in whichthe elongated body is substantially cylindrical, and in which theelongated body includes a plurality of optical fibers disposed about itscylindrical circumference.
 6. The apparatus of claim 1, in which thephotoacoustic material is of a thickness, in a direction substantiallynormal to that traveled by the acoustic energy sensed from the regionnear the distal end of the body, substantially equal to an odd integermultiple of ¼ the acoustic wavelength of the acoustic energy sensed fromthe region near the distal end of the body.
 7. The apparatus of claim 1,in which the acoustic-to-optical transducer includes at least one Bragggrating.
 8. The apparatus of claim 7, in which the acoustic-to-opticaltransducer includes an interferometer that includes a partiallyreflective first Bragg grating, a substantially fully reflective secondBragg grating, and a blazed third Bragg grating disposed between thefirst and second Bragg gratings.
 9. The apparatus of claim 7, in whichthe acoustic-to-optical transducer includes an acoustically deformablereflector located, with respect to the at least one Bragg grating, toreflect light received from the at least one Bragg grating.
 10. Theapparatus of claim 1, further including an optical coupler sized andshaped to be coupled at or near the proximal end of the elongate body tocommunicate of at least one of the first and second optical signals, andin which the optical coupler includes at least one blazed Bragg gratingto communicate at least one of the first and second optical signals, andin which the elongated body includes at least one Bragg grating near theproximal end to communicate at least one of the first and second opticalsignals with respect to the optical coupler.
 11. The apparatus of claim1, in which the optical-to-acoustic transducer and theacoustic-to-optical transducer share a common photoacoustic material forgenerating acoustic energy and forming an acoustic matching layer. 12.The apparatus of claim 1, further including: an optoelectronics module,configured to be optically coupled to a portion of the elongated body;an imaging processing module, coupled to the optoelectronics module, andwherein the user interface is coupled to at least one of theoptoelectronics module and the image processing module; a tissuecharacterization module coupled to the optoelectronics module, thetissue characterization module configured to recognize plaque; and animage enhancement module, coupled to the tissue characterization moduleand to the user interface to provide a visually distinctive display ofthe plaque.
 13. An imaging guidewire, including: an elongatesubstantially cylindrical guidewire core, including proximal and distalportions and a substantially cylindrical circumference; and a pluralityof elongate optical fibers, located along the cylindrical circumferenceof the guidewire core, each fiber including at least one of anoptical-to-acoustic transducer and an acoustic-to-optical transducer, atleast one of the fibers including an optical-to-acoustic transducer thatdirects light from a core of the at least one of the fibers to aphotoacoustic material located outside of the core of the at least oneof the fibers.
 14. The guidewire of claim 13, comprising means, locatedin the core of the at least one of the fibers, for directing lightoutside of the core and into the photoacoustic material.
 15. Theguidewire of claim 13, comprising a blazed Bragg grating, located in thecore of the at least one of the fibers, and capable of directing lightoutside of the core and into the photoacoustic material.
 16. Theguidewire of claim 13, in which the guidewire is sized and shaped to beinserted into a blood vessel, and in which the guidewire is sized andshaped to allow over-the-guidewire insertion of a catheter when theguidewire is inserted into a blood vessel.
 17. The guidewire of claim13, in which the acoustic-to-optical transducer includes at least twoblazed Bragg gratings located to cooperate interferometrically.
 18. Theguidewire of claim 13, in which the acoustic-to-optical transducerincludes a blazed Bragg grating and an acoustically-deformable reflectorlocated to reflect light received from the blazed Bragg grating of theacoustic-to-optical transducer.